A fuel cell is a power generation system that converts chemical energy of a fuel and an oxidant into electrical energy. The fuel is typically a hydrocarbon such as hydrogen, methanol, butane, etc., and the oxidant is typically oxygen. The fuel cells are divided into phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, polymer electrolyte membrane fuel cells, alkaline fuel cells, or the like. Although these fuel cells operate based on the same principle, they are different in the fuel used, operation temperature, catalyst, electrolyte, and so forth.
In a fuel cell, a membrane electrode assembly (MEA) is the most basic unit that generates electricity and consists of an electrolyte membrane and an anode and a cathode respectively formed at either side of the electrolyte membrane.
Typically, a polymer electrolyte membrane fuel cell consists of a membrane electrode assembly comprising an electrolyte membrane sandwiched between an anode (also called “fuel electrode” or “oxidation electrode”) and a cathode (also called “air electrode” or “reduction electrode”), gas diffusion layers and separators for forming a stack.
A fuel cell stack is formed by stacking tens or hundreds of unit cells where electrochemical reaction occurs. Both end plates of the unit cell or the stack are compressed by a tie rod or pressurized air in order to reduce contact resistance between components. The end plates are provided with inlet and outlet ports for a reaction gas, circulation ports for cooling water, and connectors for electric power output.
The separator is provided with a flow channel, which supplies the fuel to the anode and oxygen to the cathode, and also serves to connect the anode and the cathode of each membrane electrode assembly in series.
Electrochemical oxidation of the fuel occurs at the anode, while electrochemical reduction of oxygen occurs at the cathode. In the process, electricity is generated by the movement of electrons and heat and water are produced.
At the gas diffusion layer, diffusion of the fuel, oxygen and water occurs. Also, electrons produced in a catalyst layer are transferred by the gas diffusion layer. The gas diffusion layer is generally made of a conductive carbon material. Typically, the gas diffusion layer is made of carbon fiber, carbon cloth, carbon paper or carbon felt. Further, the gas diffusion layer of a fuel cell may further comprise a microporous layer (MPL) coated thereon. Usually, the microporous layer is made of graphite, carbon nanotube (CNT), vulcan, Ketjen black, carbon black, or the like.
The basic physical properties of the gas diffusion layer, i.e., thickness, contact resistance, gas permeability, bending stiffness, etc., are also important in the designing and manufacturing of the fuel cell stack. Individual apparatuses for measuring each of the basic physical properties are available. Also, the apparatus for evaluating the physical properties of the gas diffusion layer disclosed in Korean Patent No. 10-0902316, which was filed by the present inventors, is useful in evaluating major basic physical properties of the gas diffusion layer simultaneously under various conditions.
However, when considering mass production, the aforesaid apparatus is problematic in that, although the precision is good, a considerable amount of time is required for the evaluation. Since the degree of compression has to be changed for individual samples and the flow rate and pressure difference are evaluated for each sample, 1 to 3 hours may be required for the evaluation of one sample. It is needless to say that much more time is required when different apparatuses are used to measure the different physical properties of the gas diffusion layer.
Further, since the physical property measuring apparatus performs destructive inspection, i.e. since it requires that the gas diffusion layer in the form of a sheet or roll be broken or separated for the inspection, the gas diffusion layer cannot be used to manufacture a fuel cell after the evaluation. For an automated fuel cell stack manufacturing process, an apparatus for quality control of the gas diffusion layer should satisfy the requirements of “non-destructive inspection” and “completion of inspection in short time”. But, there is no available apparatus that satisfies them.